In the future the main objective of the communication industry will be the fusing together of the present coexisting communication networks. The driving force in this will be the demand for mobility, increased comfort for the user and the requirement of permanent availability. The aim of modern and flexible communication systems is to provide each subscriber with information services at all times, enabling him to communicate via text, sound and video wherever in the world he may be. The terminal and network environment (especially in the subscriber area) will remain very heterogeneous even in the future. For the implementation of innovative scenarios, such as mobile office, telelearning, homeworking and global information system, it is therefore necessary to provide a transnetwork audiovisual communication and transnetwork data services. An example of such a scenario is a simultaneous videoconference with mobile telephone (GSM) and PC (LAN) and narrowband/broadband terminal (ISDN, ATM).
FIG. 6 shows a selection of the plurality of existing networks, all of which are connected via a communication system. FIG. 6 also shows the respective bit rate which a network can accommodate. It can be seen that the various networks differ considerably in respect of the bit rates to be employed, the car telephone network only permitting a bit rate of less than 10 kbits, whereas a data transmission of up to 155 Mbits can be implemented by means of a superfast radio network. The Internet has no fixed bit rate since the bit rate in the Internet changes dynamically and can thus be very small when many subscribers are active whereas very high bit rates can be achieved if the Internet is relatively idle.
Devices which are capable of connecting the networks with such different bit rates shown in FIG. 6 already exist today. These devices are also called “gateways”. In principle these gateways always make it possible to connect different networks together, e.g. for video conferences. The known gateways overcome the disadvantage of today's communication systems, such as telephone network, mobile network, Internet, that they exist in parallel and only permit communication with subscribers in other networks to a limited degree.
The present gateways always depend on the respective application. For example, those which are connected between a first network with a high bit rate and a second network with a lower bit rate perform a decoding of the high bit rate data flow, then perform intermediate processing such as a format conversion, a mixing of audio signals or generation of a conference image, and encode the corresponding data flow in such a way that it can be transmitted over the second network with the lower bit rate. This step is also called transcoding or conversion. This transcoding or conversion is necessary since the audio and video standards currently used are normally matched to the bandwidth of the network so as to achieve optimal quality together with the highest coding efficiency. However, these gateways require very extensive computing resources and they are very complex, resulting in high costs both in development and also during operation (maintenance, adaptation to changing applications, etc.). Furthermore, they are very inflexible, since the connection of the networks is carried out at the application level and is thus dependent on the application.
In the following an example of a traditional gateway is discussed briefly to illustrate the application dependency. An audio encoder which operates according to a known MPEG standard is considered. This can be implemented so as to deliver an output signal with a bit rate of e.g. 50 kbits per second. This bit rate could be suitable for a local area network (LAN) which accommodates such a bit rate. If the coded audio signal is to be fed into a radiotelephone network according to the GSM standard, however, the bit rate is far too high, since mobile telephone bit rates must not exceed 10 kbits/s. A gateway is therefore needed at the interface between the LAN and the mobile radio network. This gateway must have a complete audio decoder, a complete audio encoder and also the appropriate equipment for intermediate processing. The audio encoder/decoder decodes the high bit rate signal completely and then encodes it again so as to generate an output signal which can be transmitted over the mobile radio network. Apart from the fact that the decoding, the intermediate processing and the subsequent encoding involve a very high computational effort and take time, repeated decoding and recoding, which would be necessary if there are several different networks between a transmitter and a receiver, can result in additional coding errors which arise solely because of the fact that repeated decoding and recoding are necessary. This example also illustrates the application dependency. If a video signal is to be transmitted over the LAN, this is not possible with the same gateway but only with a gateway having a video encoder/decoder. If data are to be transmitted, neither an audio encoder nor a video encoder can be employed in the gateway but rather a data transcoder. In addition it must be pointed out that if e.g. audio or video signals are processed with a new coding method, it is also necessary to replace the transcoders in all the gateways which are involved.
To summarize, current gateways require considerable computational effort, take a long time, entail high costs and are totally inflexible when it comes to reacting to changing applications. The chief cause of these disadvantages is the fact that today's gateways are completely application dependent, and therefore all the ISO-OSI layers must be passed through when transcoding. Another disadvantage which can be cited is a very inflexible bandwidth or bit rate management.